1. Field of the Invention
The present invention relates to an optical path selection switch utilizing a thermo-optic effect which is, in a wavelength division multiplexing (WDM) network, a key device of an optical cross-connect system required for high-speed and low-cost communication trunk, or an optical add-drop multiplexer (ADM) which adds an optional-wavelength signal to a WDM optical signal or drops an optional-wavelength signal off from a WDM optical signal, more specifically to a stacked thermo-optic switch fabricated based on a design concept which is to arrange optical waveguides in a stacked configuration in order to achieve a low operation power, a high extinction ratio, and low-loss characteristics in these optical switches.
2. Description of the Prior Art
For optical cross-connects or optical ADMs which are essential for WDM systems, a high-performance optical switch is indispensable.
Among optical switches, those which make connection between physically separated optical paths are referred to as space division optical switch. As a space division optical switch, an optical path selection switch using a glass or polymer material having low optical propagation loss is promising. As the switching mechanism, a thermo-optic effect is often used (see, for example, NTT R&D, vol. 44, No. 7, p. 547-552).
Thermo-optic optical path selection switches include those which utilize adiabatic propagation, those which utilize an anisotropic directional coupler (DC), and those which are based on a Mach-Zehnder interferometer (MZI).
The adiabatic type is an optical path selection switch composed of Y-branching or X-branching optical waveguides in a plane parallel to the substrate, which is known as a digital switch because stable switching operation of saturated output can be achieved when adequate refractive index differences between branches are given (Y. Hida et al., "Electron Lett." vol. 33, 1997, p. 626, or N. Keil et al., "ECOC' 96" 2.71).
The DC type is composed of directional couplers in a plane parallel to the substrate, in which optical path is switched between two states of complete coupling symmetric DC and uncoupling asymmetric DC by a thermo-optic effect (N. Keil et al., "Electron Lett." vol. 31., 1995, p. 403).
The MZI type is provided with a specific phase difference of 0 or 180 degrees using a thermo-optic phase shifter to switch the optical path.
Although the digital switches and DC type switches are stable in operation and provide high extinction ratio, they have disadvantages of large power consumption.
As an integrated matrix switch, a 16.times.16 switch which uses MZI type thermo-optic switches using a silica glass optical waveguide (Goh et al., Proceedings of the 1998 IEICE General Conference C-3-129), a 4.times.4 switch which uses DC type thermo-optic switches (N. Keil et al., Electron. Lett. vol. 31, p. 403, 1995) and the like are known. The former has problems of large power consumption, large switch circuit, and complexity of a driver circuit which are disadvantages of MZI type silica thermo-optic switch. The latter is difficult to be developed to a low-loss large-scale switch because of large propagation loss which is a disadvantage of polymer optical waveguide. Further, a delivery-and-coupling type matrix switch formed by connecting a 1.times.8 MZI type silica thermo-optic switch and a splitter by optical fiber is being studied as a practical solution (Watanabe et al., Proceedings of 1996 Electronic Information Society B-1081), which, however, has no prospect for solving the problem of complexity of fiber wiring, in addition to the problem of the unit switch itself.
As an optical ADM device, there is an example in which WDM multiplexer/demultiplexers and MZI type thermo-optic switches are formed on a single substrate using silica glass optical waveguides (Okamoto et al., Proceedings of the 1995 IEICE General Conference C-254). This uses the same number of thermo-optic switches as multiplexed wavelengths, which has the above-described disadvantage of MZI type silica thermo-optic switch and a problem in chip fabrication yield associated with upscaling.
Further, from the view point of network system, in the optical cross-connect application, stability of switching operation and thorough reduction of coherent crosstalk are even further required. In addition, though it is still before a practical application stage, estimated packaging scale of optical cross-connect in the future will never be small, and it is expected that in the future, all types of optical waveguide switches including digital type, DC type, and MZI type will be required not only for basic switching performance but also for down-sizing and cost reduction. As to the optical ADM application, since optical ADM itself is in a hierarchy closer to the user than optical cross-connects, which means a larger market size, requirements are even stricter for cost reduction and down-sizing. However, performance of optical waveguide switches at present does not meet these system requirements.
As described above, since current optical path selection switches utilizing the thermo-optic effect including both digital and DC types use optical waveguide circuits spreading out in a plane parallel to the substrate, what drives the switch is a temperature gradient in a direction parallel to the substrate. Because a heater is required to be set at a position away from a core to give a temperature gradient in this direction from the heater lying on a clad, the switch has problems of inferior heating efficiency which results in an increased operation power. Further, there is also a problem that since a temperature gradient occurs in a direction perpendicular to the substrate which has no relation to the switching operation, it is difficult to prevent degradation of switching characteristics such as an increase in optical propagation loss.
Still further, optical cross-connect switches which are now being developed are mainly of a large-sized construction combining a plurality of discrete switches with couplers in a delivery-and-coupling configuration, and high-density wiring by optical fibers reaches its limit. Yet further, as to optical ADM devices, an attempt is being made for a single chip ADM utilizing advanced design and fabrication technologies, however, in the present situation where further down-sizing is difficult in view of design, yield of chip fabrication is an important problem. As described above, in the current optical path selection switches as a whole, there are no decisive solutions in connection with down-sizing and cost reduction.
It is therefore a first object of the present invention to provide a stacked thermo-optic switch which can most efficiently utilize a temperature gradient produced by heating with a heater, is low in power consumption and loss, and high in extinction ratio.
A second object of the present invention is to provide a method for down-sizing and cost reduction in a waveguide switch construction.